Like attics and garages, disk storage systems always seem to be full. As technical development continuously moves to greater storage capacities to keep pace with increased demand, the construction and arrangement of the recording media must likewise be improved.
A conventional high capacity magnetic disk storage system typically includes a plurality of disks spaced axially on a rotationally mounted spindle. Disk storage systems attain high storage densities by utilizing the maximum number of data surfaces for recording information within a given volume. For example, an industry standard "5 1/4 inch" (approximately 130 mm) disk storage system may store 600 MB on five disks, each disk storing 60 MB on each of its two surfaces. The number of disks that can be accommodated within a given form factor, or industry standard dimensions are limited by the height of the disk stack, the spacing between the disks, and the thickness of the disks.
Higher recording densities for optical media such as CDs can also be achieved by decreasing the distance between the read head and the storage media of the disk drive. Decreasing the fly height of the read head allows for the reading of data recorded at greater densities.
However, when the fly height is decreased, the roughness or asperity of the disk surfaces is a major concern. To ensure reliable operation, asperities are generally maintained at less than 60% of the head fly height. Likewise, static build-up on the magnetic surface of the disk is kept to a minimum to reduce static-friction ("stiction") which would tend to interfere with the flying of the heads.
Conventional optical storage systems have realized high storage capacities by virtue of the fact that optical recording techniques allow recording densities substantially greater than those used with magnetic media. Optical storage systems have evolved from write-once, read-only to systems that have rewritable capabilities. The introduction of magneto-optical ("M-O") media in particular, permits dynamic and bit wise modification of information after it has been initially recorded.
In a typical M-O storage system information is recorded on the media by directing a relatively intense, focused beam of light through an optically transparent substrates (substrate incident) at a small domain in a recording layer. The recording layer is generally an amorphous, vertically oriented, magnetic film preferably made of a rare-earth transition-metal alloy, such as Tb-Fe, Tb-Fe-Co, Gd-Co, Gd-Tb-Fe, or Gd-Fe. When the domain is heated to a Curie temperature in the presence of an external biased magnetic field, the domain is magnetically polarized to align with the external magnetic field. At the end of a write pulse the heated domain cools down and regains its permanent character, storing information as a vertically oriented magnetic domain. The orientation of the domain, up or down, indicates either a logical 1 or 0.
Information is read from the recording layer by directing a less intense beam of polarized light through the substrate at the recording layer. The polarization of the beam of light is partially rotated, for example, a fraction of a degree clockwise, or counterclockwise, by the magnetically oriented domains of the recording layer by either the Kerr or the Faraday effect. In the Kerr effect the incident lightbeam is reflected at the recording layer; in the Faraday effect the polarized lightbeam passes through the material rather than being reflected from its surface. The return lightbeam essentially retraces its path to a differential detector which decodes the polarization-modulated lightbeam into bits of information.
A typical M-O storage device may store, for example, 600 MB on a single surface of one 5 1/4 inch (130 mm) disk. However, optical path restrictions, and dimensional limitations related to the optical components, that is the various lasers, lenses, mirrors, beam splitters, and prisms, have generally not allowed the use of closely spaced multi-disk arrangements in compact industry standard packages such as those which are commonly used for magnetic storage devices. Also, because of their relative size, the optical read/write components of known systems are relatively remote from the media surface, and therefore surface asperity and stiction have generally not been a major concern in optical media design.
Increased capacity optical storage systems have typically used, for example, juke-box type of mechanical means that tediously present single disks to the optical system for writing and reading information. Some optical storage systems which do use double-sided disks include means for flipping the disk over to permit access to both sides of the disks by a single fixed optical system. At an increased cost, such double-sided disk systems have been provided with duplicate optical systems to permit simultaneous access to both sides of the disks. Alternative solutions include systems where the optical system is mounted on a movable arm which can be positioned axially to select a surface, and radially to access information recorded on one of the disk surfaces.
For these, and other reasons, optical storage systems are typically more amenable with applications that use sequential access to the recorded information, such as CDs, video disks, and archival storage systems. In general, optical storage systems have not concerned themselves with minimizing the spacing between the disks, and reducing the thickness of the disks to permit the construction of a compact high capacity, random access, multi-disk optical storage device.
For optical storage systems that do use double-sided media to increase their recording capacities, the double-sided disk is typically nothing more than two single sided disks meticulously arranged with one another in a confronted relationship. Each disk in such a paired arrangement includes an optically transparent substrate with spiral or concentric relief in the form of grooves or pits formed on one surface thereof. The relief provides the disk with a pre-formatted structure for storing information and facilitates tracking of the lightbeam.
The signal recording layer is deposited on the reliefed surface of each disk. The two, thus fabricated, single sided disks are then carefully aligned and bonded to each other with the signal recording layers being directed inwardly. The outwardly facing surfaces of the disks are usually coated with a protective layer, or lubricant to minimize corrosion, and wear and tear ("tribology"). The disadvantages of such a double-side disk are numerous. Since this type of disk uses two substrates, the total thickness of the finished disk is at least twice the thickness of the substrate, increasing the total space required for using such a disk in a multi-disk configuration. The increased weight of the disk increases the time and power required to spin the disk up to operational speed, and subject the drive motor to undue wear and tear. For substrate incident media, the substrate must be formed by a carefully controlled injection molding process to produce good transmittance and birefringence characteristics. The requirement for precise alignment and registration of the two substrates furthermore decreases process yield while increasing process cycle time. In addition, the surface topology of known optical disks is generally too rough to be compatible with low-flying air-bearing read/write heads. Excessive asperity and stiction increase the probability of fatal collisions if used with a low-flying air-bearing read/write head, as are commonly used with magnetic storage systems.
Accordingly, the known medium for optical disk storage systems increase the cost of assembly, require a carefully controlled injection molding process, and moreover, are not compatible with compact high capacity, multi-disk storage devices using low-flying air-bearing read/write heads.
It will also be appreciated from the foregoing that there is room for further improvement to the media proposed in the prior application. Specifically it would be desirable, that read-only storage media be provided which is compatible with magneto-optical storage media as defined in the prior application cross-referenced above, so that either rewriteable media, or read-only media can be used in the same disk storage system.
A common method of manufacturing of a read-only optical storage media generally includes the steps of forming a microstructure on the information bearing surface of a substrate. The microstructure generally is in the form of depressions or pits arranged in a spiral or concentric tracks. The microstructure can be formed by conventional embossing or molding techniques.
For read-only media, the microstructure usually has a depth which is approximately equal to 1/4 the wavelength of the light beam that is used to read the data. A reflective layer, generally a metal alloy, is formed on the substrate to reflect the lightbeam. The microstructure increase the distance that the lightbeam travels during its incident and return paths by 1/2 wavelength. Destructive interference causes the intensity of the lightbeam to vary. The return light beam which is reflected at the substrate surface is sensed and decoded to yield digital signals corresponding to the data recorded in the microstructure of the disk.
In order to prevent damage to the reflective layer or the microstructure of the substrate during handling or by moisture, a protective layer is formed on the reflective layer. Prior art read-only disks have used a lacquer, a polycarbonate resin, or a thin glass plate.
However, such relatively thick protective layers are not compatible with low flying air bearing read heads. Therefore, the read heads and optical components such as focusing lenses are placed at a substantial distance from the recording surfaces of the disk. In a typical prior art read-only disk, the head may be as far as 0.001 centimeters from the surface, severely limiting the recording densities used to store information.
Low flying, low mass, air-bearing read heads, also known as sliders, such as are commonly used with magnetic media, typically fly at a distance of about 0.00001 centimeters form the disk surface. The relatively lack of asperity of the prior art read-only disk surfaces have generally not allowed the use of low flying sliders to achieve greater storage densities.
Therefore, it is desirable to provide optical media which can be used with a disk storage device which: is easy to manufacture with automated assembly equipment; uses minimal and inexpensive materials; is compatible with low-flying air-bearing read/write heads, and provides for an increased storage capacity without increasing the overall size of the system. Furthermore, it is desirable, for such media to be compatible with both read-only and rewritable optical storage systems, be they fixed or removable.